How the concrete is worked out
Concrete fills a cylinder: the hole, minus the gravel at the bottom, minus the post standing in it. Divide that volume by what one bag yields and you have bags per post. Multiply by the number of posts for the job.
Bags = (π × (hole diameter ÷ 2)² − post area) × (hole depth − gravel) ÷ bag yield
The yields come from the manufacturers' data sheets. Quikrete gives 0.375 cu ft for a 50 lb bag of Fast-Setting Concrete Mix, and 0.30, 0.45 and 0.60 cu ft for its 40, 60 and 80 lb bags of Concrete Mix. The calculator rounds up once, for the whole job, not once per hole: a bag you open for one post and do not finish goes into the next.
A worked example
Ten 4x4 posts (3.5 in actual size) for a 6 ft fence, in 42 in holes 10.5 in wide with 6 in of gravel at the bottom:
- Concrete depth: 42 in − 6 in = 36 in.
- Hole area: π × 5.25² = 86.6 sq in. Post area: 3.5 × 3.5 = 12.25 sq in. Concrete area: 74.3 sq in.
- Volume per post: 74.3 × 36 = 2,676 cu in, which is 1.55 cu ft.
- 50 lb fast-setting bags: 1.55 ÷ 0.375 = 4.13 bags per post.
- Ten posts: 15.49 cu ft ÷ 0.375 = 41.3, so buy 42 bags.
The same job takes 26 bags of 80 lb Concrete Mix, 35 of 60 lb or 52 of 40 lb. The 42 in depth and the 10.5 in width are what the fence post depth calculator recommends for a 6 ft fence; start there if you have not settled on a hole yet.
How many fence posts per bag of concrete?
Usually less than one. A standard 4x4 in a hole three times its width takes about two to five and a half bags of 50 lb fast-setting mix for fences 3 to 8 ft high, so one bag sets a fraction of a post. The picture changes for thin metal posts: a 1 5/8 in chain link line post in a 6 in hole with 24 in of concrete takes 0.97 bags, so roughly one post per bag. A 5x5 vinyl post in a 12 in hole with 30 in of concrete takes 4.08 bags.
Bags per post by hole size
For a 4x4 post and 50 lb fast-setting concrete (0.375 cu ft per bag), by the depth of concrete in the hole. Add your gravel to get the hole depth.
| Concrete depth | 8 in hole | 10 in hole | 12 in hole | Sakrete, 10 in hole |
|---|---|---|---|---|
| 12 in | 0.70 | 1.23 | 1.87 | 1.25 |
| 18 in | 1.06 | 1.84 | 2.80 | 2 |
| 24 in | 1.41 | 2.46 | 3.74 | 2.5 |
| 30 in | 1.76 | 3.07 | 4.67 | 3 |
| 36 in | 2.11 | 3.68 | 5.60 | not listed |
The last column is Sakrete's own chart for its 50 lb Fast Setting Concrete Mix in a 10 in hole, which does not name a post size. It sits within a quarter of a bag of the calculator in every row. Quikrete's data sheet is more optimistic: it says two 50 lb bags set a 3 in post in a 9 in hole 30 in deep, but the same arithmetic gives 2.53 bags for that hole.
Quikrete's own online calculator comes out higher than both, for a different reason: it treats the post as round and does not subtract any gravel, so it fills the whole hole with concrete.
How much water?
Fast-setting mix is poured dry into the hole around the braced post, then soaked. Quikrete's data sheet and project sheet both give about 1 gallon of water per 50 lb bag, and the data sheet asks for holes deeper than 30 in to be filled in lifts of 30 in or less. Sakrete's no-mix method runs the other way: fill the hole a third full of water first, then add the mix.
Regular Concrete Mix is mixed before it goes in. Quikrete's data sheet starts at 3 pints of water for a 40 lb bag, 4 pints for 60 lb and 6 pints for 80 lb, and allows up to 4.5, 7 and 9 pints. The calculator shows the starting amount.
Fast-setting or regular mix?
Fast-setting mix saves the mixing, and it is the product both Quikrete and Sakrete show for setting posts. Regular 80 lb bags give more concrete per bag and fewer bags to carry, at the cost of mixing each one. The volume in the hole is the same either way; only the bag count changes.
What else to plan
The gravel volume in the results is loose gravel for the bottom of every hole. To count posts for a whole fence line, use the fence post calculator. The fence calculator puts concrete, posts, rails and boards on one list.